Machines such as, for example, loaders, excavators, dozers, motor graders, and other types of heavy equipment use multiple actuators supplied with hydraulic fluid from an engine-driven pump to accomplish a variety of tasks. These actuators are typically pilot controlled such that, as an operator moves an input device, for example a joystick, an amount of pilot fluid is directed to a control valve to move the control valve. As the control valve is moved, a proportional amount of fluid is directed from the pump to the actuators. For cost and efficiency reasons, the machine's engine may be too small to drive the pump and supply a maximum amount of pressurized fluid that could be demanded by an operator at any given time. Thus, it may be possible for the operator's demands, if fully satisfied, to stall the machine's engine under some conditions. That is, the amount of power required from the engine to drive the pump, as demanded by the operator, may exceed an output capacity of the engine, thereby causing the engine to stall.
One method of selectively reducing the load on an engine under stall conditions is described in U.S. Pat. No. 7,165,397 (the '397 patent) issued to Raszga et al. on Jan. 23, 2007. The '397 patent describes a hydraulic control system that automatically reduces or eliminates the hydraulic load on an engine in response to the capacity of the engine being overcome by the hydraulic load. Specifically, the hydraulic control system includes a pilot supply circuit, which supplies pressurized pilot fluid to a plurality of implement joysticks. An anti-stall valve is located within the pilot supply circuit to selectively block the supply of pressurized pilot fluid from the joysticks. The pilot supply circuit applies a pressure differential to opposite sides of the anti-stall valve dependent on engine speed. As engine speed reduces toward stall conditions, the pressure differential correspondingly reduces and blocks pilot fluid flow to the implement joysticks. When the pressure differential falls below a certain value, the pilot flow is turned off and remains off until engine speed recovers. In addition, movement of the anti-stall valve also relieves pilot fluid pressure in communication with the implement joysticks to a low pressure drain. The hydraulic control system thereby provides quick, smooth, and potentially total removal of hydraulic load on the engine, while holding the positions of the hydraulic functions when engine speed drops. This enables a machine designer to select an engine size that will be efficient for most operations, without concerns for occasional different or combined operations that produce engine speed decreases and stalls.
Although the hydraulic control system of the '397 patent may reduce the likelihood of stalling an engine due to hydraulic overloading, its usefulness may be limited. Specifically, because the anti-stall valve only turns the pilot fluid off and holds the positions of the hydraulic functions, there may be situations where hydraulic operation is completely halted when only a reduction in operation was necessary. In addition, there may be conditions where adjustments to the anti-stall operation may be beneficial, such as when the operator desires to run the engine at less than full speed. Under these conditions, the hydraulic control system of the '397 patent must simply be rendered non-operational by way of an override valve.
The disclosed control system is directed to overcoming one or more of the problems set forth above.